Understanding the molecular mechanisms that control cardiac cell cycle progression and exit is essential and required for effective cardiac regeneration therapy. The majority of mammalian cardiomyocytes undergoes terminal differentiation and rapidly switches from hyperplasia to hypertrophy resulting in binucleation soon after birth. However, a small population of differentiating but immature and mononucleated cardiomyocytes, similar to prenatal ones, may retain a proliferative ability and could enter the cell cycle to produce new cardiomyocytes. This project aims to determine the roles of canonical Wnt/ b-catenin signaling and the brahma-related gene 1 (Brg1) chromatin remodeling complex in cardiac cell cycle progression and exit during postnatal heart development and acute myocardial infarction. Our preliminary data suggest that adenomatous polyposis coli (APC), a tumor suppressor frequently mutated in colon cancer, is a potential master switch that turns off cardiomyocyte proliferation, but how APC regulates the cardiac cell cycle remains an unanswered question. The central hypothesis to test is if the decline of Wnt/b-catenin activity leads to Brg1 downregulation after birth and is directly responsible for the transition from cardia proliferation to hypertrophy. We further propose that Brg1 is sufficient and required for cardiac cell cycle progression. Using gene therapy, cardiac-specific genetic targeting and mutagenesis, we will create models for gain- and loss-of-function in Wnt signaling and the Brg1 chromatin remodeling complex. With these models, we will pursue these specific aims: 1) To determine the roles of APC and b-catenin in CMs during the postnatal hyperplasia-hypertrophy transition of CMs; 2) To investigate the role of Brg1 in b- catenin signaling during postnatal heart development and after acute myocardial infarction. Our ultimate goal is to determine if manipulating b-catenin signaling and the Brg1 chromatin remodeling complex can stimulate cardiac regeneration and repair after acute and chronic myocardial damage.